This invention relates to a thickness monitoring system for use in combination with a layer forming device operable to form a layer on a substrate in a hollow space. It should be noted here that the layer forming device may be a sputtering device, an evaporation device, an ion plating device, or the like, although the sputtering device will mainly be described throughout the instant specification.
A sputtering device of the type described comprises a pair of electrode members which are opposite to each other in a hollow space and one of which serves to support a substrate thereon and the other of which comprises a conductive plate and a target plate of a material to be sputtered onto the substrate. An a.c. voltage is often supplied between the electrode member pair to cause sputtering of the target material towards the substrate. As a result, a desired layer of the target material is deposited on the substrate. Such an a.c. voltage has a high frequency selected between several hundreds of kilohertz and several tens of megahertz.
In the meantime, it is preferable with a layer forming device, such as a sputtering device, that a thickness of the layer must always be monitored by the use of a thickness monitoring system during the sputtering. The thickness monitoring system often comprises a quartz crystal exposed in the hollow space to a flow of the target material and coated with a monitoring layer of the target material, simultaneously with the deposition of the layer on the substrate. The quartz crystal covered with the monitoring layer has a variable resonance frequency dependent on a variation of weight of the monitoring layer, as known in the art. Therefore, it is possible to detect the thickness of the layer deposited on the substrate, by monitoring a variation of the high frequency produced from the quartz crystal.
When the quartz crystal is applied to the sputtering device driven by such high frequency a.c. voltage, the quartz crystal is adversely affected by the high frequency. As a result, it is difficult to accurately monitor the thickness with the quartz crystal because a result of measurement is inevitably accompanied by an error.
Preferably, such a thickness monitoring system is available to various types of sputtering devices each of which is driven by the high frequency a.c. voltage.